The nuclear magnetic resonance apparatus corresponds to an apparatus for irradiating an electromagnetic wave in a state of arranging a sample below a static magnetic field, observing and analyzing a free attenuation signal from an atomic nucleus in the sample generated thereby, and executing a structural analysis of a material in the sample. In recent years, it is expected to be applied particularly to an organic polymer such as a protein or the like.
The conventional NMR apparatus is structured such that a superconducting magnet is arranged perpendicular to an installation surface, and a probe is inserted to the superconducting magnet from a vertical direction to the installation surface. On the other hand, a high sensitive NMR apparatus obtained by applying an improvement to the conventional NMR apparatus is described in patent documents 1 and 2 (JP-A-2003-329755 and JP-A-2003-329756). The high sensitive NMR apparatus is provided with a pair of split superconducting magnets arranged in a horizontal direction in the installation surface, and has a low temperature probe inserted to the split superconducting magnet from a horizontal direction and including a transmitting probe coil and a receiving probe coil.
In order to obtain a high sensitivity in the NMR apparatus, it is effective to cool the receiving probe coil to a very low temperature. In order to operate stably for a long time, there can be considered a method of cooling the receiving probe coil by circulating a cooling medium, however, a cooling performance of a refrigerating machine has a limit. Further, it is hard to obtain a temperature stability with respect to a heat generated in the transmitting probe coil at a time of irradiating a radio wave. In addition, since both of a specific heat and a coefficient of thermal conductivity become lower in most materials under a very low temperature state, it becomes further hard to obtain a high-level temperature-stability.
In order to obtain a temperature stability, there is a method utilizing a latent heat of a helium, however, a high cooling performance is required in the refrigerating machine, and a refrigerating machine utilizing a Joule-Thomson effect is frequently used. This kind of refrigerating machine having a high cooling performance is hard to be handled. A cooling apparatus using a general Gifford McMahon refrigerating machine (a GM refrigerating machine) rather has an advantage for a user although a cooling performance is inferior to the above.
Patent document 3 (JP-A-10-332801) describes a structure of an NMR probe which is operated at a very low temperature while using the cooling apparatus such as the GM refrigerating machine. However, in the case that the cooling apparatus is structured by one GM refrigerating machine and two or more countercurrent heat exchangers, it is hard to supply a cooling medium at a stable temperature with respect to a load fluctuation of the refrigerating machine. The load fluctuation in this case is constituted by an electric heat generation of the transmitting probe coil. If no countermeasure is applied, the temperature of the probe coil fluctuates at a certain cycle and a time constant.
In the structure of the single turn superconducting magnet and the low temperature probe as described in the patent document 3, the sensitivity is saturated together with a reduction of the temperature, and it is impossible to obtain a benefit caused by an improvement of the sensitivity on the basis of the cooling at a certain temperature or less. Accordingly, the receiving probe coil is cooled at a temperature in the vicinity of 20 K. On the other hand, in the low temperature probe having the split type superconducting magnet and the solenoid type receiving probe coil described in the patent documents 1 and 2, the sensitivity is described as being improved to a lower temperature. Therefore, the receiving probe coil is cooled in the vicinity of 5 K.
FIG. 9 shows a comparison between the temperature and the sensitivity in the solenoid type and split type probe coils. As illustrated, a sensitivity (S/N) of the split type is high in the low temperature region.
The probe of the NMR apparatus is sensitive to a fluctuation of the state. With respect to a room temperature and a temperature of a sample, a fluctuation width of 0.01 or a level in proportion thereto is often required. In the high sensitive NMR apparatus described in the patent documents 1 and 2, a temperature stability of the receiving probe coil is particularly important.
In the case of cooling the receiving probe coil in the very low temperature state by one of low temperature side stages of the refrigerating machine, if a temperature of a part of the circulating cooling medium is increased due to a heat generation of the transmitting probe coil or the like, it takes a long time to average a temperature of an entire of the circulating cooling medium. The cooling medium exchanging heat with the receiving probe coil has a time change with an external waviness. This phenomenon is not negligible in the case of the high sensitive measurement even in the conventional NMR apparatus. Further, in the ultrahigh sensitive NMR apparatus described in the patent documents 1 and 2, it is not allowable.
In the case that the conventional cooling apparatus is applied as it is to the low temperature probe of the ultrahigh sensitive NMR apparatus, the receiving probe coil has a risk in the temperature stability as mentioned above, and the reduction of the sensitivity caused thereby is calculated.
The temperature change of the probe coil and a resonance circuit portion forming a pair therewith causes a change in a circuit constant or the like, and exerts an influence on an acquired signal. In particular, there are generated a change in a resistance of a wiring on the basis of the temperature change, and a change in a capacity of a condenser. Further, there are generated a change in a resonance frequency, a change in a Q value, and a change in an input impedance.
In order to structure the high sensitive NMR apparatus, it is preferable that the Q value of the resonance is higher. The higher a purity of a material is, the more the material is affected by the temperature change. In the case that a superconductive material is used, the property is sensitively changed by the temperature. In addition, a very uniform magnetic field is required in the NMR apparatus, however, since a magnetism of the constituting material is changed by the temperature at the very low temperature, there is generated a problem relating to a magnetic homogeneity.